This application is based on Japanese Patent Application No. 2002-81041, filed on Mar. 22, 2002, the entire contents of which are incorporated herein by reference.
A) Field of the Invention
The present invention relates to a semiconductor device, and more particularly to a semiconductor device with reduced parasitic capacitance between two adjacent impurity diffusion regions formed in a semiconductor substrate.
B) Description of the Related Art
FIG. 11A is a cross sectional view of a photodiode which is one kind of a photo sensor. On the surface of a p-type silicon substrate 100, an epitaxial layer 101 made of n-type silicon is formed. On the surface of the n-type epitaxial layer 101, a field oxide film 102 is formed to define a plurality of active regions.
In one active region (an active region in the central area of FIG. 11A), a plurality of n-type cathode regions 103 are formed mutually spaced apart by a certain distance. Between two adjacent cathode regions 103, a p-type separation region 104 is formed. The surface of the active region 104 in which the cathode regions 103 and separation region 104 are formed is covered with an antireflection film 105.
In each of the active regions (active regions on the right and left sides in FIG. 11A) adjacent to the active regions in which the cathode regions 103 are formed, a p-type anode lead region 106 is formed. The bottom of the anode lead region 106 reaches the p-type silicon substrate 100.
The cathode region 103 and p-type silicon substrate 100 constitute a photodiode. The p-type silicon substrate 100 functions as the anode of the photodiode.
Such photodiodes are widely used as an optical pickup device to be used with a photoelectric conversion device typically an optical disc such as a DVD and a CD, as a photo sensor having a photoelectric conversion function. A photo sensor to be used with an optical disc is desired to operate at high speed as the wavelength of a laser beam becomes shorter. In order to realize stable high-speed operation, it is desired to reduce various types of parasitic capacitances in the photo sensor and prevent leak currents.
The photo sensor shown in FIG. 11A has parasitic capacitances between the cathode region 103 and adjacent anode region 106, between the cathode region 103 and p-type silicon substrate 100 and between the cathode region 103 and separation region 104. These parasitic capacitances are desired to be reduced in order to ensure stable high-speed operation.
FIG. 11B is a cross sectional view of a conventional photo sensor whose parasitic capacitance is partially reduced. Between the cathode region 103 and anode lead region 106, a trench 108 is formed through the field oxide film 102, the trench reaching the surface layer of the p-type silicon substrate 100. A silicon oxide film is formed on the bottom and inner sidewalls of the trench 108, and polysilicon is filled in the trench 108.
A p-type high impurity concentration region 109 is formed in a region of the p-type silicon substrate 100 and n-type epitaxial layer 101 in contact with the trench 108. This p-type high impurity concentration region 109 prevents leak current from flowing via the bottom of the trench 108.
Since the thin silicon oxide film having a dielectric constant lower than that of silicon is formed on the sidewall of the trench 108, parasitic capacitance between the cathode region 103 and anode lead region 106 can be reduced.
Although the parasitic capacitance between the cathode region 103 and anode lead region 106 of the photo sensor shown in FIG. 11B can be reduced, the parasitic capacitances between the cathode region 103 and p-type silicon substrate 100 and between the cathode region 103 and separation region 104 cannot be reduced.
Since the p-type high impurity concentration region 109 is formed around the trench 108, parasitic capacitance is newly formed between the cathode region 103 and p-type high impurity concentration region 109.
An object of this invention is to provide a semiconductor device with reduced parasitic capacitance between two impurity diffusion regions having opposite conductivity types.
According to one aspect of the present invention, there is provided a semiconductor device comprising: an underlying substrate having at least a surface layer made of semiconductor of a first conductivity type; a first layer formed on or over the underlying layer and made of semiconductor having a resistance higher than a resistance of the surface layer of the underlying substrate; a first impurity diffusion region formed in a partial surface region of the first layer and doped with impurities of a second conductivity type opposite to the first conductivity type, the first impurity diffusion region not reaching a surface of the underlying substrate; a second impurity diffusion region of the second conductivity type disposed in the first layer and spaced apart from the first impurity diffusion region in an in-plane direction by a certain distance, the second impurity diffusion region reaching the surface of the underlying substrate; and a first separation region disposed between the first and second impurity diffusion regions and comprising a trench formed in the first layer and dielectric material disposed at least in a partial internal region of the trench.
The first layer having a high resistance is disposed between the first impurity diffusion region and underlying substrate. Parasitic capacitance between the first impurity diffusion region and underlying substrate can therefore be reduced. Since the dielectric material is disposed in the trench constituting the first separation region, parasitic capacitance between the first and second impurity diffusion regions can be reduced.